There is a need for a non-invasive and continuous bedside monitor for brain oxygenation for adult human patients. Near-infrared spectroscopy (NIRS) is an optically based technique that could meet such demand. However, NIRS instruments have yet to achieve their initially expected potential of becoming routine clinical monitors complementary to pulse oximetry. One major challenge is to develop an optical transducer probe and a corresponding algorithm to solve the problem of interference from extracerebral tissue (skin, scalp, and skull) in adult NIRS applications. Another major challenge is to develop an algorithm that will be able to absolutely quantify the optical signals to derive quantitative variables or indices that are of clinical significance. Our goal is to resolve these challenges to develop an NIRS monitor that will determine absolute brain hemoglobin oxygen saturation on adult patients. The primary objectives of this SBIR proposal are: 1) Design and construct an NIRS monitor for use on adult human patients; 2) Determine the optimum adult NIRS probe configuration to minimize extracerebral interference 3) Evaluate and validate novel adult NIRS algorithms that further minimize extracerebral interference and determine absolute brain oxygen saturation.